In general, in order to purify purification target components such as hydrocarbon (HC), carbon monoxide (CO), nitrogen oxides (NOx), particulate matters (PM) and the like included in an exhaust gas of an internal combustion engine such as a Diesel engine, an exhaust gas purification system including an exhaust gas purification apparatus having each of catalyst devices such as an oxidation catalyst device (DOC), a particulate trap device (CSF, SCRF and the like), a selective reduction-type catalyst device (SCR), a lean NOx trap catalyst device (LNT) and the like has been used.
There is an exhaust gas purification system having a lean NOx trap catalyst device in the exhaust gas purification apparatus and configured to purify nitrogen oxides included in an exhaust gas. In the exhaust gas purification system for decreasing NOx, at a state (lean state) where oxygen is excessively included in the exhaust gas, like a lean burn engine and a Diesel engine, NOx is temporarily occluded in the lean NOx trap catalyst device, and when a NOx occlusion amount increases, NOx cannot be all occluded with the lean NOx trap catalyst device and thus a downward slip amount is likely to increase, the exhaust gas is periodically controlled to a rich air-fuel ratio, so that NOx occluded in the lean NOx trap catalyst device is discharged and reduced. The operations of occlusion, discharge and reduction are repeated, so that a NOx conversion efficiency by the lean NOx trap catalyst device is maintained.
In NOx regeneration processing of performing rich reduction of NOx occluded in the lean NOx trap catalyst device, as a trigger for shifting to the rich control, only two threshold values of an occlusion amount threshold value of NOx in the lean NOx trap catalyst device and a NOx concentration threshold value (or a NOx conversion efficiency threshold value), when a NOx concentration sensor is provided in an exhaust passage, are used. That is, when a NOx occlusion amount occluded in the lean NOx trap catalyst device becomes equal to or greater than a preset NOx occlusion amount threshold value and when a NOx concentration detected by the NOx concentration sensor becomes equal to or greater than a preset NOx concentration threshold value (in other words, the NOx conversion efficiency is equal to or less than a preset NOx conversion efficiency threshold value), the control is shifted to the rich control.
In this case, however, since the control is shifted to the rich control, irrespective of an operating pattern of the engine, the control may be shifted to the rich control of the exhaust gas at a timing, which is not always optimal for improvement on the NOx conversion efficiency and fuel consumption. As a result, the NOx conversion efficiency and fuel consumption are not stable.
For example, even when the NOx occlusion amount is equal to or greater than the NOx occlusion amount threshold value and the conversion efficiency is lower than the NOx conversion efficiency threshold value, if the operating state of the engine is a low-speed and low-load state, the NOx discharge amount is low, so that the control is not necessarily required to be shifted to the rich control. However, the control is shifted to the rich control under current control.
Regarding the above problem, as disclosed in Japanese Patent Application Publication No. 2002-81335A, an exhaust purification apparatus of an internal combustion engine has been suggested which has a function of narrowing an operation region of a lean air-fuel ratio upon acceleration operation of the engine when a relation between a total NOx discharge amount and a traveling time period of the vehicle lies on a total exhaust amount increase-side higher than a predetermined relation, considering that a regulated value by NOx discharge regulation is a total NOx discharge amount with respect to a predetermined traveling distance, and which is configured to effectively suppress the total NOx discharge amount and to effectively recover NOx occlusion performance by increasing an opportunity of NOx reduction.
In the above exhaust purification apparatus of an internal combustion engine, it is controlled whether or not to allow the lean operation in which the NOx discharge amount increases upon the acceleration operation of the engine. Thereby, when there is a margin in the total discharge amount by the relation between the total NOx discharge amount and the traveling time period of the vehicle, the operation by the lean air-fuel ratio is allowed even during the acceleration operation, so that the fuel consumption is improved and the total NOx discharge amount is managed.